Why is it 60i

To me it seems like Progressive and Interlaced are effectivly the same thing. However, Interlaced has a different scanning method, i.e: fields and scanned twice as fast, odd (down) then even (up) fields; progressive gets scanned once, all down.

So 30p = 60I because 30 progressive frames scanned as interlaced makes 60 fields, which to me would be more like 30I then 60. hmmm?? 60 fields yes, 60 interlaced frames... well since they're not deinterlaced it wouldn't be 60. It would be 30 Interlaced frames and 60 fields.
 
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TimurCivan said:
the reason they went with interlacing at alll was to eliminate the TV screen from flickering. 30 htz, isnt fast enough to trigger persistence of vision. ( neither is 24p BTW, they actually flash each frame of film twice so youre seeing 48 flashes of light per sec) so to help keep the TV from flickering the engineers in the 20's and 30's settled on an interlaced signal, to make the screen flash twice as fast.

i can actually see the flicker in PAL tvsets. its at 50 HTZ, and its just detectable to me. so i have trouble watching TV in europe, it gives me a headache.

( the set not the programming, italian TV is where life begins... especally after 10PM..... )
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Hold the phone. I thought the reason we have 60i was because old televisions couldn't handle 30 true frames per second because of limitations to the tube, but COULD handle 60i. I also thought the reason it was 60i instead of PAL's 50i was becasue of the 60MHz cycle for electriciy here, vs the 50MHz cycle in Europe, which all ties in to the differences in voltages and whatnot as well.

Am I totally of the mark here or does anyone care to clarify?
 
Rapier_100a said:
Hold the phone. I thought the reason we have 60i was because old televisions couldn't handle 30 true frames per second because of limitations to the tube, but COULD handle 60i. I also thought the reason it was 60i instead of PAL's 50i was becasue of the 60MHz cycle for electriciy here, vs the 50MHz cycle in Europe, which all ties in to the differences in voltages and whatnot as well.

Am I totally of the mark here or does anyone care to clarify?
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I read that somehwere too. but like there was a problem with flicker. So interlacing i guess killed two birds, ya know. And yes the difference in PAl is the power frequency. I suppose the framerate had to be divisible by the power frequency. so the machine could operate smoothly.

Shoot i may have read wrong sources. We need some Wikipedia action.....
 
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PunkMetalDrummer17 said:
To me it seems like Progressive and Interlaced are effectivly the same thing. However, Interlaced has a different scanning method, i.e: fields and scanned twice as fast, odd (down) then even (up) fields; progressive gets scanned once, all down.

So 30p = 60I because 30 progressive frames scanned as interlaced makes 60 fields, which to me would be more like 30I then 60. hmmm?? 60 fields yes, 60 interlaced frames... well since they're not deinterlaced it wouldn't be 60. It would be 30 Interlaced frames and 60 fields.
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No. They are not the same thing.

When you shoot 60i, you're recorded 60 discreet moments in time, and for each of those moments, you're recording half a picture.

When you shoot 30p, you're recording 30 discreet moments in time, and for each of those moments, you're recording a whole picture.

Because in 60i you're recording 60 separate moments in time, you have more temporal information, so motion is smoother and more natural.

Because in 30p you're switching from whole picture to whole picture and not half-picture to half-picture, you do not suffer from the Kell factor and you preserve the full resolution of the shot, whereas with interlaced, you never have better than about 77% of the resolution of progressive.
 
TC said:
Wait, since when is there a 30i?

Now I'm confused. Where the hell is Mr. Green?
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"30i" is an archaic term which usually means the same as what we now commonly call "60i." "60i," however, is the more accurate term (even though it's actually ~59.94i).

Avid still has what it calls a "30i" timeline. That's where NTSC DV lives on planet Xpress Pro.
 
David is, of course, exactly correct.

The problem comes from the unfortunate custom of referring to NTSC video as 29.97 "frames" per second. There's no such actual thing as a "frame" of NTSC video. Two fields do not make up a "frame", two fields are TWO FIELDS. Two different things. Calling NTSC "29.97 frames per second", or calling PAL "25 frames per second" has unnecessarily confused the issue over the last 50+ years.

Think of 60i as if it was 60p, 60 frames per second, but each frame is only half-resolution. 60i looks more like 60p than it looks like 30p, because the motion sampling is the same.

You can't just say "field one plus field two = a frame", because as David pointed out, each field is a different moment in time. It's a different picture. It's not half of a 30p picture, it's its own different picture. That's why when you combine two fields in your NLE, or in PhotoShop or whatever, you frequently get that interlaced "combing" artifact -- anything that was moving will show clearly that it was taken from two different moments in time.

So NTSC isn't 29.97 "frames" per second, because they're not actually frames in the context that we think of frames (as in, individual pictures). They're two separate pictures, which are grouped together out of convenience for the editing program more than from any aspect of actually belonging together. On an NTSC (or PAL) television, each field will spend as much time on-screen with the field that preceeded it, as it will with the field that follows it. If nothing is moving at all, then yes the two fields can be combined to create something that looks like a frame, but any motion at all will completely ruin that process.

So, again -- you can think of 60i as sort of a hack way of doing 60p. 60i has the temporal feel of 60p, but it's half-resolution because each field is only the even lines, or only the odd lines, of an image. It was done this way in the beginning of television because they just didn't have enough bandwidth to transmit a full 60 frames. So splitting the frames into "fields" is basically a form of analog compression; each "frame" is cut in half, but because it's done on a line-by-line basis, it doesn't feel like it's been chopped entirely in half.

If you de-interlace footage, what that means is you attempt to turn each field into its own full frame. Image data is interpolated between the lines to create a new "field", one that has the same motion characteristic as the field it was interpolated from. So de-interlaced NTSC means that half the fields are just thrown away, dropping your data stream from 60 motion samples per second down to 30 motion samples per second. But NTSC still requires 60 fields per second, so the missing fields are interpolated from the remaining fields, so you end up with 60 fields again, made from only 30 motion samples, so the footage takes on the motion characteristics of 30p. 30P would be about twice as high resolution, but de-interlaced 60i would have the same motion characteristics as 30p.

But 30p != 60i. Just look at the footage on the screen and you'll see there's clearly a difference.
 
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